1. Botanic gardens maintain collections with the intention to preserve diversity. This includes both live plants and seed collections.
2. Seed banks. Many countries and international organizations maintain refrigerated collections of seeds of thousands of plants, both wild species as well as crop types. The major collection on Svalbard Island is the best known one.
3. Perhaps you could consider the national parks attempts to preserve diversity.
I fully agree with the response of James Des Lauriers. In the present situation of biodiversity, with many conservation problems aorund the world (between them climate change, destruction of habitat and forest fires), it's clear that seed banks and live plant collections (conservation ex situ) and establishment of protected areas complemented by direct conservation measures in the field (conservation in situ) are very important. Many threatened plants have been saved from extinction adopting such measures.
Of course I fully agree with the replies of previous colleagues, but this applies to where man has gone and where there is human management. In many cases the excellent human management proved capable of conserving resources. However, the truly quality resources remain in areas untouched by humans and far from any human management.
Before conservation, it is important to understand that what is to be conserved? Due to limited taxonomic studies, it has become difficult to identify and document the species of area under investigation. Gardens can conserve few species because of availability of fixed land. The collective efforts at all level using variety of approaches are required to promote conservation of floristic at larger scale.
By creating more protected areas in select biodiversity hot spots around the world, national parks, conservancies, and game reserves to protect and conserve plant biodiversity. Further, regular assessment of genetic diversity of plant species and establishing gene banks to preserve seeds of important plant species, and finally community awareness and education of the importance of plant diversity and its role in the environment and for human benefits.